This invention relates to thermoplastic dental impression trays. More particularly, it involves a method of making a thermoplastic dental impression tray which maintains its molded structure to a high degree of accuracy.
To form a final impression of a dentulous or edentulous alveolar ridge, in the course of producing a male model of desired oral tissues, conventional practice involves the use of commercially available preliminary impression trays of use with either the maxillary or the manibular alveolar ridges and continguous tissues, in either dentulous or edentulous cases. These trays are available in a range of sizes so that one which makes an approximate fit with the patient's mouth area may be chosen for use. They have a cross-section characterized by a base with two normally extending walls so that they will surround the ridge of which an impression is to be formed. These stock trays are typically formed of aluminum or other material that may be bent or trimmed at the edges to provide the fit for an individual patient. Impression material is placed in the tray and a female cast of the required area is formed. This cast is then typically used to form a male model of the patient's mouth section either in the dental office or in a separate dental laboratory. The laboratory or dental office then uses this male model to form a custom final impression tray out of a plastic material which closely conforms to the individual patient's oral cavity. The thus formed custom molded tray is then shipped back to the dental practitioner to use the custom tray to form a final impression of the required area by placing a high definition, settable, impression material such as alginate, silicone, or polysulfide or the like, into the impression tray. The tray is then pressed to cause the impression material to closely surround the desired area. As known in the art, it is necessary that the spacing between the tray and the tissues must be on the order of only several millimeters to obtain an accurate final impression. Since the custom tray already anatomically conforms to the patient's tissues, it is generally not subject to any further deformation.
It can be seen that this conventional practice typically requires two procedures and is relatively expensive. Additionally, the stock impression trays may cause discomfort to the patient because of its relatively gross fit. Similarly, inaccuracies creep into the procedure because of the multiple transfer steps.
A number of alternative means for forming final impressions have been proposed to overcome these recognized inaccuracies of the conventional procedure. For example, several forms of stock trays have been devised which may be shaped exteriorly of the mouth to improve the accuracy of the initial impressions. U.S. Pat. No. 3,473,225 to Deuschle et al discloses such a stock tray which is formed of a thermoplastic material which can be molded outside of the mouth by the dentist after slight heating in order to improve its fit. Similarly, U.S. Pat. No. 3,653,705 to McAdoo discloses a thermoplastic dental impression tray which may be cut and then stapled back together in order to better conform to the patient's mouth.
Several arrangements have been proposed for elimination of the initial impressions altogether and formation of a final custom impression tray directly in the patient's mouth. U.S. Pat. No. 1,995,709 to Kinsley discloses an impression tray formed of a woven metal matrix with a soft pliable covering that may be contoured directly to the patient's mouth. German Pat. Nos. 715,041 and 1,297,811 are representative of the use of thermoplastic materials in the form of dental impression trays and the like. In German Pat. No. 715,041 there is disclosed a dental impression tray which can be heated by placing it in boiling water so that it can be bent in accordance with the mouth proportions. However, this and other prior art attempts to solve the problems associated with conventional processing all have one fatal flaw: after they have been molded, they do not retain their molded structure to a sufficient degree of accuracy. Once they have been deformed they have a tendency to return to their original shape. This tendency is referred to as memory or recovery rate. It is evident that if the memory of the tray is such that it has a tendency to return to its original shape, the accuracy of the impression is impaired rendering it almost totally useless. For this reason, commercial acceptance of these known thermoplastic impression trays have not been forthcoming.
It is believed that the prior art devices have not been acceptable because their glass transition temperatures were too low. The term "glass transition temperature" is the temperature at which a thermoplastic material changes from a glassy brittle state to a rubbery state characterized by a change in stiffness of several orders of magnitude. In this rubbery state, the thermoplastic material may possess relatively stiff properties. However, it will still have too high a recovery rate for commercial acceptability.